The present invention relates generally to the operation of polymer electrolyte fuel cells, and, more particularly, to the operation of polymer electrolyte fuel cells at high voltages.
One of the challenges for polymer electrolyte fuel cells (PEFCs) is to maintain high energy conversion efficiency, particularly for transportation applications. Transportation applications may require a cell to operate under xe2x80x9ccruisingxe2x80x9d conditions at high voltages, e.g., in the range of 0.75 to 0.85 V. At these voltages, the burden of performance is primarily on the cathode because the oxygen reduction reaction (ORR) at the cathode is kinetically sluggish. In comparison, the fuel reaction, i.e., the oxidation of H2 at the anode, introduces negligible performance losses in this voltage range.
Various strategies have been proposed to improve cathode performance. These strategies include catalyst layer composition and catalyst layer structures. Increasing the Pt-catalyst loading at the cathode will increase performance at any voltage; but this xe2x80x9cbrute forcexe2x80x9d approach is only partially effective. Our laboratory studies have found that, even at operational fuel cell voltages as high as 0.9 V, PEFC cathodes show mass transport limitations where the cell currents reach plateaus with increasing Pt loading and the value of the limiting currents depends on catalyst composition. For example, for cathodes containing 20% Pt/C, the plateau appears at a loading of about 0.4 mg Pt/cm2 at 0.9 V.
Another approach is to use Pt-alloys rather than pure Pt as the cathode catalyst. Indeed, alloys containing nominal compositions of Pt3Cr have been used for many years in phosphoric acid fuel cell cathodes. Other Pt-alloys comprising metals such as Mn, Fe, Co, and Ni also enhance the oxygen reduction reaction so that overall PEFC performance is improved.
In all cases, however, Pt-based cathode catalysts appear to have a fundamental limitation when PEFCs are operate at high voltages. Experiments strongly suggest that, at these voltages, there is an inherent Pt activity loss for the ORR due to adsorption onto Pt surfaces of oxygenated species from water. FIG. 1 shows a cyclic voltammogram of a fuel cell cathode containing 0.2 mg Pt/cm2 in a carbon supported catalyst. The anodic currents starting at 0.75 V correspond to the reaction: Pt+H2Oxe2x86x92Pt+OH+H++exe2x88x92. When the fuel cell is forced to operate at voltages of about 0.75 V and higher, a partial Ptxe2x80x94OH coverage is induced on the catalyst and the number of Pt active sites decreases.
Over time, the active Pt surface area on the cathode becomes insufficient to sustain the ORR at the initial rate. As a consequence, the initial current drops while the cell voltage is maintained at the high level. Within the first 60 minutes at 0.8 V, a cell may drop its current as much as 50% of the original output, as shown by the lower curve of FIG. 2. Polarization curves, widely used as diagnostic tools to evaluate fuel cell performance, do not predict this operational shortcoming. Performance losses in the same time range are not observed when the cell is operated at constant voltages lower than 0.6 V. In this case, a steady state condition is reached within one or two minutes.
PCT Application WO 98/42038, xe2x80x9cFuel Cell with Pulsed Anode Potential,xe2x80x9d published Sep. 24, 1998, teaches that fuel cell power losses arising from CO poisoning of Pt anode catalyst can be reduced by periodically increasing the anode potential by shorting the anode or by connecting the anode to a positive external voltage. The cathode is shown connected to ground and does not appear to experience any potential change. The anode voltage pulse parameters proposed to overcome CO poisoning of the anode catalyst (pulse width of 10 to 200 ms, pulse amplitude of 700 mV, pulse frequency of 0.01 to 0.5 Hz) do not change the polarization potential of the cathode and are not sufficiently long to remove OH from the Pt active surfaces.
In accordance with the present invention, an operating strategy has been developed to overcome this performance degradation arising from Ptxe2x80x94OH coverage of active Pt sites and maintain an average current close to the initial value even when the fuel cell is operated for long times at high voltages.
Various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention is directed to a method for operating a fuel cell at high voltage for sustained periods of time. The cathode is switched to an output load effective to reduce the cell voltage at a pulse width effective to reverse performance degradation from OH adsorption onto cathode catalyst surfaces. The voltage is stepped to a value of less than about 0.6 V to obtain the improved and sustained performance.